1. Field of the Invention
The present invention relates generally to cellular mobile communications systems, and more specifically to a power saving technique for a mobile unit frequency synthesizer for a TDMA (time division multiple access) cellular communication system.
2. Description of the Related Art
As illustrated in FIG. 1, a prior art frequency synthesizer for use in a TDMA cellular mobile communications system includes a direct digital synthesizer 10 that supplies reference pulses to an initial phase alignment circuit 11 to which the output of a frequency divider 16 is also applied to establish phase alignment with the reference pulse. The phase aligned signals are input to a phase comparator 12 where their phase difference is detected and fed via a switch 13 to a loop filter 14. A voltage-controlled oscillator 15 supplies a VCO output to the frequency divider 16 at a frequency variable with the output of the loop filter 14. A power-saving controlled DC voltage is supplied to the power-draining units of the frequency synthesizer such as DDS 10, phase comparator 12 and frequency divider 16 to periodically turn off their power supplies to reduce their energy consumption.
When the mobile unit is in a standby mode in a host cell site area, the power supply to the transmitter is continuously turned off and the power supply to the receiver is periodically turned on and off so as to enable it to monitor the control slot of the TDMA frame of the host cell site. The power-saving controlled voltage is synchronized with the power saving operation of the receiver. During a turn-off period, the switch 13 is turned off to operate the frequency synthesizer in an open-loop mode by feeding the VCO 15 with a voltage developed by the loop filter 14 during the previous turn-on period and currently maintained by the loop filter.
During a talking mode, the transmitter and receiver are alternately turned on during assigned transmit and receive slots of the TDMA frame and turned off during other time slots. The receiver is further turned on during an idle slot of the TDMA frame to enable it to be switched to the channel of an adjacent cell site for a possible hand-off.
Since the mobile unit is required to make a channel search and return to the current channel within a short period of time, the prior art mobile unit cannot turn off the power-draining parts of its frequency synthesizer at a high speed during talking modes. Therefore, the power saving feature of the synthesizer is used only during standby modes.